1. Field of the Invention
The present invention relates to a transflective LCD, and more particularly to a transflective LCD having a dual common-electrode structure.
2. Description of Related Art
Please refer to FIG. 4, a cross sectional view of one pixel in one conventional transflective LCD has a front substrate (60), a liquid crystal layer (70) and a rear substrate (80). The front substrate (60) has a flat transparent conductive film used as a common electrode (61). The rear substrate (80) has a thin film transistor (TFT) (81), a storage capacitor (82) and a reflective layer (83) that is above the TFT (81) and the storage capacitor (82). Since the pixel is in the transflective type LCD, each pixel has a reflective region (R) corresponding to the reflective layer (83) and a transmission region (T). The reflective region (R) reflects the ambient light (L1) and the transmission region (T) allows the back light (L2) to pass through the rear and front substrates (80, 60). With reference to FIG. 5, since each pixel is driven only by a driving voltage but has the reflective and transmission regions (R, T), each pixel accordingly has two different separate V-T curves (RV-T, TV-T).
As disclosed the U.S. Pat. No. 6,806,929 entitled “Transflective LCD driven by bi-gamma curve” issued to Chen et al. ('929 hereinafter), with reference to FIG. 8, since the cell gap (d1) of the reflective region differs from the cell gap (d2) of the transmission region, the transmission region has a phase difference nearly double that of the reflective region. Reduction in cell gap of the reflective region has been adopted in the prior art to minimize the phase difference. However, it is difficult to adjust the cell gaps (d1, d2) of the reflective and transmission regions to fit the LCD's operation mode. Therefore, the '929 patent provides two separate TFT driving circuits in each pixel and uses two gamma curves from a data driving unit to fit two V-T curves. Each TFT driving circuit has a thin film transistor, a storage capacitor and a liquid crystal capacitor. The two TFT driving circuits respectively correspond to the reflective and transmission regions to supply two suitable driving voltages to the reflective and transmission regions. The fabricating cost of the LCD provided by the '929 patent reasonably increases and the effective area of each pixel is reduced.
In a Taiwan patent (TW502237) a method of adjusting the V-T curve of the LCD is disclosed. According to the method, the V-T curve is changed by adjusting the voltage supplied to the common electrode. With reference to FIG. 6, a voltage waveform diagram shows a voltage waveform of the data line (Vd), a voltage waveform of the common electrode (VCOM) and a voltage waveform of a liquid crystal capacitor (VLC). In general, the voltage of the common electrode should be kept in a constant level. However, the '237 patent adjusts the voltage level of the common electrode to change the voltage of the liquid crystal capacitor.
With further reference to FIG. 7, multiple V-T curves (A, B, C, D, O) can be produced based on different voltages supplied to the common electrode. Therefore, the V-T curve can be changed by supplying different voltage to the common electrode.
Therefore, the present invention provides a transflective LCD having a dual common-electrode structure and uses adjustable voltages supplied to common electrodes to produce two V-T curves similar to each other.